In a traditional wavelength division multiplexed (WDM) network, individual traffic channels are carried by distinct wavelengths of light that travel together along a single optical fiber for considerable distances until some of the channels are switched at a switching node. In order to provide the requisite switching functionality, a switching node is typically designed to convert incoming optical signals to electrical signals, to decode the contents of the electrical signals, to regenerate the electrical signals, to switch the regenerated electrical signals in accordance with a connection map, and then to re-convert the switched electrical signals back into optical signals. For this reason, switching nodes are expensive and are typically only located at a limited number of strategic points in the network.
As a result, traditional networks often contain long fiber segments shared by the same group of channels, with no possibility of diverting individual channels from the group or coupling additional channels to the group until a switching node is reached. However, it is sometimes highly desirable to divert an individual channel from the group or to couple an additional channel to the group before a switching node is reached. To this end, the art has seen the development of the optical add/drop multiplexer (OADM). This component makes it possible to add channels to (and drop channels from) a “main” optical path existing along a network segment which might otherwise not have access to switching functionality in a traditional network. An especially advantageous feature of OADMs is that channels can be added and dropped without the need to perform electro-optical or opto-electronic signal conversion.
However, the widespread use of OADMs along a main optical path comes at a price. Specifically, because optical amplifiers along the main optical path have a gain that varies with wavelength and because the fiber itself has an attenuation which varies with both path length and wavelength, the variety among the different paths taken by different channels will cause each channel to experience a different degree of performance degradation. As a result, the optical signal to noise ratio (OSNR) and bit error ratio (BER) of a given channel at its end point will vary widely from one channel to another, which is an unacceptable condition from a network provider's point of view.
Moreover, the use of OADMs at arbitrary points along a main optical path also degrades the performance of in-service channels due to “traffic hits” caused by the sudden addition of a channel to the main optical path.
Clearly, therefore, it would be desirable to be able to equalize the performance of each channel travelling along its own distinct portion of a main optical path. However, despite the availability of techniques for equalizing multiple channels that share the same end-to-end route between two switching nodes, the industry still lacks a technique to equalize multiple channels that travel along diverse subsets of a main optical path.